My guess would be that no, no overdrilling/filling is done by the builder. If they do, they should be shouting it to the hills, because they would be the only production boat builder I know of to do so. In other words, it's pretty "normal" for a production builder to just drill, caulk, and install. I do have a production boat from the early 60's that actually did put thickened resin in where hardware, etc. went; but from what I have seen, it is very, very rare. Definitely something worth mentioning (shouting out, really) if they are doing it.
As to whether it is necessary.... in my mind it's a resounding YES, but then I have had to re-core several boats, and it is absolutely no fun - plus the materials are expensive. I've also seen a good number of boats essentially abandoned (often at boatyards) because paying someone else to do the re-core work cost so much more than the boat was worth. Most of these boats didn't develop severe problems until they were ten or more years old though; it does take time.
Will every boat that doesn't have the drill/overfill have a problem? No, absolutely not. That's because there are other ways to keep water out of the core. For example, keeping a trailered boat stored under cover, so that it's really only "out" a small percentage of the time. Generally, moisture problems sort of seep or creep in - they don't happen overnight or even in a week or month (usually). Or, a person could be very diligent about re-bedding (i.e. caulk) hardware fittings and other core penetrations. Another thing that helps is good backing blocks because they help to spread the load and avoid crushing the core when tightening the fasteners. Once it is crushed you can get a sort of "well" around a given item that can funnel water in. A backing block helps to prevent this; an epoxy annulus is even better (still with appropriate backing block). So it's not that good caulk and careful bedding can't keep water out; it's just that how do you know when it's time to re-bed? With a "solid" area there, you'll get a leak into the interior of the boat to tell you it's time, but that water will not get into the core (which can otherwise start before you get a noticeable leak into the boat).
It's not that wood and/or foam themselves are so fragile. After all, wooden boats are in water 24/7 and can last for centuries. You can toss a piece of foam out the car window and it will last for ... decades(?) But it's the fact of having wood -- or gaps -- that are "trapped" inside a non-vented area. If water gets in then it can stay there and "stew," which is not a good thing. Also, the bond between core (wood or foam) and the fiberglass is very important. It's what gives the laminate its planned-for strength (i.e. one firm boat vs. "three floppy ones" if it were to debond). So even if you have a core that is less prone to rot (i.e. treated/balsa, foam), it's still not a good thing to get water in, because it can find its way into (inevitable) gaps in the laminate and start the de-bonding process. Also, water can actually sort of be "sucked" in and around once you have more than one "opening." That's because as the boat heats up, vacuums and things are created (sorry, not good with the technical terms, but it's like the boat can "pant" from temperature changes, or flexing, and then water that can get in can be pulled/pushed around and through any gaps).
These are not the actual boat, and so (one hopes) they were just not built as well, but for example one of my factory floorboards was ruined by a combination of things mentioned above (they are molded fiberglass and cored, like a boat). To explain: I purchased the floorboards from a different C-Dory 22. They had mostly been kept in the owner's garage (he was a fisherman and liked the deeper cockpit), but I guess had been used a bit. One day I was looking them over, and I saw a very, very thin hairline crack of about 8" long on the underside. I got tools out expecting to widen it, perhaps take out a touch of damp core, and then repair it with a strip of fiberglass tape and epoxy. Ha ha.
First I found out that a construction flaw had not only probably greatly contributed to the crack, but had also allowed water to flow within the construction sandwich, thus wetting a much larger area. That flaw was that where the board was "stepped" (on the underside), two separate pieces of core had been used. But the gap between them was not filled at all, so there was a big "channel" running the length of the board (inside the cored area, unseen). Then, the laminate schedule (which was mostly weak mat) was not adjusted in this area, so there was a weak spot just itching to crack. It did crack, a bit of water seeped in (must have been one of the rare times the previous owner of the floorboards had them in place, as I had not had them in water at all), and then it freely ran up and down the gap between the two sections of core, allowing it to get saturated. Then another problem caused the cascade that resulted in me finding about 75% of the core saturated: The kerfs in the core were not filled, for the most part. The kerfs run in a grid pattern and make the core into ~2" squares, held together with scrim for handling during construction. They make the core so it will flex and mold to curves (not necessary in the floorboards, but many times kerfed core is just used for everything - I know I do). "Best practice" is to fill all those kerfs with thickened epoxy. These were not, and that allowed the water to run all over the floorboard utilizing this hidden "grid" of pathways. The other floorboard had a smaller version of the same crack, and the same construction (from what I can see), but had not yet become wet.
But again, is every boat that is not overdrilled/filled going to self-destruct? Absolutely not. First of all, many trailered boats aren't even outside that much. Also, there are other ways to keep water out of the core, which is the ultimate goal (good caulking and diligent maintenance). And even when the above are not true, it takes a while for problems to develop - it's not typically something that happens overnight. Many boats are being happily sailed with some wet core; they are doing fine for now and may for years and years to come. And it is extra work to overdrill/fill, so that's time and money (although it's easier to accomplish at the point of build than later, as you can just plan to lay the boat up with solid fiberglass in those areas - you don't then have to go back and dig out core, etc.). I don't know of any production builders who do. There very well may be some, but it's certainly not common. So C-Dory is not "worse" in this way (presuming they have not changed methods recently).
Sunbeam
PS: It's not uncommon for a builder to use both foam and/or balsa in the same boat. Sometimes it's a matter of whatever is handy goes in. Sometimes (probably not in C-Dorys) it's a matter of specific properties. For example, as I understand it, balsa is "stronger" in certain high-load/larger panel situations - essentially it can be stiffer than foam (again, non proper technical terms here). So a sportfisher with large-area panels that are going to be heavily stressed might be engineered with a very specific core material in mind (which might be balsa). On the other hand, it may be that the website is just a combination of other/older websites, and so language from both has been blended (some balsa; some foam). Many boatbuilders and boatyards are more into building boats than websites.
PPS: It's somewhat of a misnomer that one not need worry about water getting in if a thing is foam cored --- it's still not good to have water in the core/laminate interface.
